A recent electrophotographic image forming apparatus has been expected to be durable even from a viewpoint of the reduction of environmental burdens. One of the factors hampering the expected durability is degradation of an image due to a stain on a charging roller.
The charging roller is mainly employed as a charging unit in the electrophotographic image forming apparatus. To achieve a high image quality, in most cases, a photosensitive element is charged by superimposing a direct current and an alternate current, and applying the superimposed current to the charging roller.
Conventionally, there has been developed various technologies for protecting the charging roller from being stained. As for the development of the charging roller itself, for example, a surface roughness and a friction coefficient of the charging roller are reduced, and thereby preventing the charging roller from being stained with a toner or the like.
As for the development of a cleaning member for cleaning the charging roller, for example, according to a conventional technology disclosed in Japanese Patent Application Laid-open No. 2004-251974, a brush roller is provided to clean a surface of the charging roller, and a surface roughness of the charging roller is set to be 25 μm or less, and thereby preventing the charging roller from being stained with a foreign substance including the toner. However, only the setting of the surface roughness of the charging roller is not enough to prevent the charging roller from being stained with the foreign substance, so that it is not possible to achieve a sufficient durability.
According to a conventional technology disclosed in Japanese Patent Application Laid-open No. 2006-64953, a sheet-like stain-resistant member is arranged to have contact with the charging roller, and a surface roughness of a surface of the sheet-like stain-resistant member is set to be larger than that of the surface of the charging roller. However, with this configuration, the stain-resistant member slidingly moves with being rubbed against the surface of the charging roller, so that a stain on the charging roller is stuck when the charging roller is used for a long time, and thereby making it difficult to achieve a sufficient durability.
According to a conventional technology disclosed in Japanese Patent No. 3317748, it is configured that a contact pressure P1 between a charging roller and a photosensitive drum is larger than a contact pressure P2 between the charging roller and a cleaning blade, a friction coefficient μ1 between the charging roller and the photosensitive drum is smaller than a friction coefficient μ2 between the charging roller and the cleaning blade, and a rubber hardness of the charging roller is larger than that of the cleaning blade. With this configuration, the cleaning blade slidingly moves with being rubbed against the surface of the charging roller, so that a stain on the charging roller is stuck when the charging roller is used for a long time, and thereby making it difficult to achieve a sufficient durability, in the same manner as the technology disclosed in Japanese Patent Application Laid-open No. 2006-64953.
In this manner, by providing the cleaning roller, it is possible to slow a rate of getting the charging roller stained, i.e., it is possible to extend a life span of the image forming apparatus. However, it was found that once an additive agent of the scraped toner, the toner, a lubricant, and the like adhered on the cleaning roller are saturated, they are rubbed into the surface of the charging roller due to a slight slipping between the cleaning roller and the charging roller. Therefore, it is necessary to develop a configuration capable of preventing a slipping between the cleaning roller and the charging roller.
An object of the present invention is to prevent an occurrence of the degradation of an image due to “a stain on the charging roller”, as one of the factors hampering a sufficient durability of the image forming apparatus. In other words, an object of the present invention is to improve a cleaning effect on the charging roller.